This invention relates generally to scanning image tubes employed in video cameras. More particularly, the present invention relates to devices which have a photoconductive sensor-target with a relatively large area and capacity suitable for applications such as X-ray imaging.
Video camera tubes to which the present invention relates employ a low velocity electron beam (LV beam) and a very high resistivity photoconductive sensor-target. As used in this application, low velocity electron beams generally refer to electron beams which strike the sensor-targets with sufficiently low velocities that secondary emissions are not produced.
The photoconductive type sensor-target employed in video camera tubes has two functions. First, the sensor detects irradiating photons. Second, the target functions to hold electrons deposited on its inner target surface by the electron beam as the LV beam scans out a raster. In low velocity no gain tubes to which the invention relates, an electron beam scans the inside surface of the sensor-target in a raster scanning fashion. The electrons strike the sensor-target surface with low energy and the electrons are uniformly deposited across the surface of the sensor-target to drive the potential across the surface to approximately the potential of the electron gun cathode which is conventionally, but not necessarily, at ground. Modification of the charge on the sensor-target resulting from projection of a optical image onto the input sensor surface of the image tube generates a time varying video signal as the LV beam scans through the raster. The image is obtained on a successive pixel basis as electrons lost at the target surface through image formation are replaced by the scanning electron beam.
The sensor function of the sensor-target is accomplished by an insulating, photoconductive material responsive to the incident radiation to be imaged which has a resistivity on the order of 10.sup.12 ohms per centimeter. The high resistivity is required in order to maintain sufficient electron charge storage and electron immobility on the inner scanner surface of the sensor-target during the raster scanning. On exposure of the sensor-target to incident radiation from an image of an object or specimen, such as, for example, a chest X-ray, a resulting charge flow through the sensor-target medium results in electrons being lost from the scan surface. The changing radiation intensity which comprises the incident image results in a proportional electron loss on the target surface. The electronic image is thus read out by the scanning electron beam.
Various photoconductive targets suitable for use in conjunction with low velocity electron beams have been identified. Miller in Proc. Phys. Soc., Vol 50, 374 (1938) disclosed photoconductive targets which employed selenium, zinc sulphide, cadmium sulphide, thallium sulphide, antimony sulphide, zinc selenide, cadmium selenide and zinc telluride. H. G. Lubszynski U.S. Pat. No. 2,555,091, P. Weimer U.S. Pat. No. 2,654,853, P. Weimer U.S. Pat. No. 2,687,484 and R. Goodrich U.S. Pat. No. 2,654,852 disclose devices which employ a photoconductive sensor-target tube. The noted Weimer and Goodrich patents disclose devices which employ a uniform layer of antimony trisulphide as the sensor-target material in a device known as the Vidicon.TM. tube. L. Heijne U.S. Pat. No. 2,890,359 discloses a tube which employs a lead oxide sensor-target and which became a prominent television tube marketed under the Plumbicon.TM. mark. The tube disclosed in the Heijne patent incorporates a p-i-n junction which exhibits less lag than the previous conventional tubes.
There have been a number of systems advanced which employ video tubes in connection with X-ray imaging. Mackay U.S. Pat. No. 4,852,137 discloses a method and an apparatus wherein X-rays and high energy photons are transmitted through a specimen and visible light emitted by the secondary source is detected by a cooled slow readout CCD. While there have been successful small diameter tubes, e.g., on the order of a few inches in diameter, attempts to transform small diameter tube technology to large diameter tube configurations have encountered significant technical problems, including difficulties in employing sensor-target layer compositions such as lead oxide on a large diameter scale, unacceptable stray capacitance and associated unacceptable noise characteristics. Partial solutions to some of the technical problems have been advanced in the conventional small diameter tube context. Van de Polder et al U.S. Pat. No. 4,059,840 discloses the use of a signal electrode comprised of vertical, multiple strips to deal with the problem of stray capacitance in television tubes. Heretofore scaling conventional small diameter tubes to large diameter tube configurations which are more suitable for high energy imaging, such as X-ray and gamma ray imaging, has presented formidable technical obstacles. In summary, large area ionizing radiation sensitive LV-type tubes have not found widespread success due to an absence of a suitable electronics designs and an absence of suitable sensor-target materials.
A number of earlier video tube designs were disclosed by the co-inventor of the present invention in Nudelman U.S. Pat. No. 4,794,635. The latter patent discloses a number of devices and features directed to overcome the deleterious effects of high capacitance in large area raster scanner imaging tubes employed in video cameras. The use of CsI as one component of a sensor-target is also disclosed. Among the novel features disclosed in the patent are the use of a plurality of selectively oriented signal electrode strips and a multiple layer, solid state structure which provides a displaced electron layer sensor-target (DELST) for imaging with and without photoconductive gain and with and without intensifier gain. In the DELST structure, various features are selected and combined to provide a scanner suitable for various applications and which satisfy various specifications such as speed, spatial resolution, dynamic gain, and sensor-target characteristics as well as cost.
Conventional LV-type tubes which are scaled to larger areas exhibit excessive lag due to the the high beam resistance and the large charge storage capacity. Excessive storage capacity ordinarily result from increasing the size of the target, increasing the dielectric constant of the sensor-target medium and/or decreasing the thickness of the sensor-target medium.
As an additional design consideration, a high DQE which approaches 100% is a desired feature for an optimal scanner tube. A DQE of 100% represents a sensor having quantum efficiency wherein the output noise is only limited by the photon noise at the input where the image is initially projected. In practice, photon noise should dominate all other electronic sources of noise, and the modulation transfer function (MTF) of the tube and its dynamic range must match the imaging requirements for an application.
The large area imaging required in diagnostic radiology, nuclear medicine, non-destructive testing and other technologies can be successfully accomplished by a detailed reconsideration of the underlying rationales for the materials, structures and functions of conventional video tube designs. For such large area applications, real time imaging may not be required. Moreover, the spatial resolution requirements are not as demanding as the requirements for conventional television camera tubes. These relaxed constraints are important in connection with successfully overcoming the very significant capacitance problems associated with large area video tubes.